FIGURE 2.

Arabidopsis ferric reductases and transporters that contribute to cellular iron homeostasis. Evidence suggests that FRO6 functions to reduce Fe³⁺ to Fe²⁺ at the cell surface of leaf cells; Fe²⁺ is subsequently transported across the membrane via an unknown transporter(s), while other unknown transporters may be involved in the uptake of Fe³⁺. Iron is then trafficked to a set of intracellular organelles. Chloroplasts utilize a reduction-based mechanism for iron acquisition via FRO7, whereas FRO3 and FRO8 may serve an analogous function in mitochondria. PIC1 serves as a chloroplast iron transporter while rice MIT mediates iron uptake by mitochondria. Although there is not yet any evidence for vacuolar metalloreductases in plants, it is known that VIT1 is important for iron uptake by vacuoles while NRAMP3/4 function in vacuolar Fe efflux.